In atomic absorption spectroscopy a line emitting light source such as a hollow cathode lamp emits a measuring light beam containing a resonance line of an analyte the amount of which in a sample is to be determined. The sample is introduced into an atomizer and atomized therein such that the sample forms an atomic vapor in which the analyte contained in the sample exists in the atomic state. The measuring light beam passes through this atomic vapor and impinges upon a photoelectric detector. The measuring light beam is attenuated by the atomic vapor as a function of the number of atoms of the analyte which are contained in this atomic vapor and absorb the resonance line.
The atomizer may be a flame produced by a burner. In this case, a solution of the sample is continuously sprayed into a mixing chamber of the burner by means of a nebulizer for producing a continuous absorption signal.
The atomizer instead may be an electrothermal atomizer such as a graphite tube atomizer. A metered quantity of sample is introduced into the electrothermal atomizer and atomization is effected by passing a strong current through the electrothermal atomizer. The atomizer is thereby heated to atomization temperature and atomizes the sample. There is thus obtained a transient, peak-shaped signal.
In any case, the detector generates a signal which is indicative of the amount of the analyte contained in the sample. This detector output signal can be calibrated in terms of, for example, "concentration" in the case of the flame atomizer, or "quantity" in the case of the electrothermal atomizer. In the latter case, the transient signal is usually integrated with respect to time and this time-integrated signal is used for determining the quantity of the analyte contained in the sample.
In order to correct for "background absorption" due to, for example, absorption by non-atomized molecules in the atomic vapor, it is known to apply a magnetic field to the atomic vapor. Because of the Zeeman effect, the resonance lines of the atoms in the atomic vapor, then, are shifted and the resonance line of the measuring beam is no longer absorbed. Thus, in the presence of the magnetic field, there will be no specific absorption of the measuring beam by the atoms of the analyte and only the background absorption is measured. In the absence of the magnetic field, both the specific absorption and the background absorption become effective to attenuate the resonance line of the measuring beam. A signal indicative of specific absorption alone and corrected for background absorption can thus be obtained by simple arithmetic.
For quantitative determination, the instrument has to be calibrated. Samples containing known amounts of the analyte are supplied to the atomizer. Theoretically, the absorption to which the measuring beam is subjected, should follow Lambert-Beer's law: The logarithm of the detected light intensity in the presence of the analyte referenced to the unattenuated light intensity or, if desired, referenced to the detected light intensity attenuated by a blank, i.e. the absorbance, is proportional to the number of atoms of the analyte in the atomic vapor. Thus, if the absorbance is plotted versus the amount of the analyte contained in the sample, a linear graph passing through the origin ought to be obtained. From such graph, the amount of the analyte contained in the sample can be determined. It has been found, however, that frequently and particularly so during use of the electrothermal atomizer, the graph of the absorbance versus the amount of the analyte is non-linear. Specifically, the graph is increasingly curved at high amounts of the analyte in the sample and asymptotically approaches a maximum absorbance value in non-Zeeman measurements. When utilizing the aforenoted Zeeman background correction, the graph even passes through a maximum and drops again at still larger amounts of the analyte (roll-over).
Therefore, it is one object of the invention to provide a method and apparatus of the initially mentioned type and which method and apparatus permit obtaining substantial linearization of the measured absorbance as a function of the analyte quantity so that the obtained signal is substantially proportional to the analyte quantity substantially through entire range of measurement.
It is a further and highly important object of the invention to provide a method and an apparatus of the initially mentioned type and which method and apparatus permit obtaining substantially linearized calibration graphs particularly but not exclusively through substantially the entire measurement range of time-integrated absorbance using electrothermal atomizers in the absence as well as in the presence of Zeeman background correction.